如何将 Future[Either[Throwable,T]] 展开为 Either[Throwable,T]

Question

I have a function ( myFunc ) in scala that gives Future[Either[Throwable,T ]] . Now I need to unwrap and get Either[Throwable,T ] out of it and pass to as an input parameter to another function ( anotherFunc ).

def myFunc(input: String): Future[Either[Throwable, HttpResponse]] = {
  ....
}

def anotherFunc(response: Either[Throwable, T]) # signature

anotherFunc(myFunc("some string"))

Normally we use map to transform a Future but thats not helping me here

myFunc("some string").map { _ =>
            anotherFunc(_)
          }

This causes problem with the return of the block from where I am calling.

Answer 1

You can't unwrap the value of a Future because a Future represents the result of an asynchronous computation that may or may not be available yet. By default, futures and non-blocking, encouraging the use of callbacks instead of typical blocking operations.

What you can do is either:

• use combinators such as map , flatMap , filter to compose futures in a non-blocking way.
• register a callback using the onComplete method, or foreach if you want to call a callback only when the Future completes successfully.
• block the main thread using Await.result , if this is absolutely necessary, although is discouraged. If you want to transform the Future result or combine it with others, you should opt for the 2 previos non-blocking ways mentioned.

That being said. These are the preferred approaches:

  def anotherFunc[T](response: Future[Either[Throwable, T]]) = {
    response.onComplete {
      case Failure(exception) => // process exception
      case Success(value)     => // process value
    }
  }

  def anotherFunc2[T](response: Future[Either[Throwable, T]]) = {
    response.map {
      case Left(exception) => // process exception
      case Right(value)    => // process value
    }
  }

Then you can do:

  anotherFunc(myFunc("some string"))
  anotherFunc2(myFunc("some string"))

NOTE : If you can't change the signature of anotherFunc[T](response: Either[Throwable, T]) then just do:

  myFunc("some string").map(anotherFunc)